The present invention relates in general to a mold for shaping concrete parts such as for example concrete pipes, shaft wall rings and the like. The molding device is of the type which includes a hollow mold core, at least one centering ring located in the interior of the mold core and being fixedly attached thereto, a core jarring machine, centering means including an annular centering member mounted on the periphery of the core jarring machine and having a cylindrical outer surface fitting a cylindrical inner surface of the centering ring such that the latter together with the mold core can be inserted on the centering member, hydraulic clamping means arranged between the core jarring machine and the centering means to exert radial pressure against a movable clamping member and displace the same into engagement with the inner surface of the centering ring.
In a prior art molding device of this kind the centering ring mounted on the core jarring machine consists of individual holders arranged in circumferential direction of the machine in a spaced relation one to another and each having an outer surface. Each holder supports a pressure member which is arranged for a reciprocating movement in a radial passage in the housing of the core jarring machine and is urged by a resetting spring into the radial passage. The rear side of the pressure piece is provided with pistons reciprocating in corresponding cylinders in the form of cylindrical bores in the holder. The cylinders are supplied via pressure conduits with a hydraulic fluid acting on the free end faces of the pistons. Upon pressurizing of the fluid the pistons are displaced into the assigned cylinders whereby the pressure piece is shifted in radial direction against the force of the resetting springs. Such a hydraulic clamping device for clamping and releasing the mold core on a centering jarring machine can be provided on several locations on the centering jarring machine where unbalances might occur. This hydraulic core clamping device makes it possible to rapidly replace a mold core on the jarring machine for another one.
The disadvantage of such known clamping devices is the considerable installation space required for its component parts. Another substantial disadvantage of the prior art clamping device is its high manufacturing costs which render the whole molding device more expensive. In addition, the component parts of the known clamping device are relatively complicated. The installation of numerous movable parts necessitates many seals which apart for increased manufacturing and installation costs require additional maintenanace costs. The seals are subject to wear and therefore leakage in the course of service life must be taken into account. The resetting springs for biasing the pressure piece into its starting position is another element requiring further manufacturing costs and is prone to breakage leading to an inoperativeness of the clamping device. Also the sliding elements performing a relative movement to the adjoining parts are also exposed to wear causing non-uniform distribution of clamping forces which in turn may result in the blocking of the whole clamping device.